1. Field of the Invention
This invention concerns an arm unit of a jack and, particularly, it relates to an improvement of an arm unit for use in a pantograph type jack having four arms combined in a rhombic link structure for jacking the weight of automobiles.
2. Description of the Prior Art
As has been well-known, a pantograph type jack usually comprises a pair of lower right and left arms pivoted to a lower bearing member of a grounded base and a pair of upper right and left arms pivoted to an upper bearing member of a load receiver, in which the ends of the respective arms opposed to each other are rotatably pivoted to constitute a vertically deformable rhombic link. An actuation lever having threads formed therealong is rotatably screw-coupled to the rhombic link along the horizontal orthogonal line, so that the load receiver may be displaced vertically by rotating the actuation lever thereby deforming the rhombic link.
The jack of the type described above is generally adapted such that the mating ends of the paired right and left arms are brought into rolling contact with each other under frictional resistance in the bearing portions so that the movements of the four arms are made equal to each other. For ensuring the rolling contact, the ends of the paired right and left arms attached to the bearing members are meshed with each other through a gear coupling in the prior art. However, since this structure requires steel material of a great thickness for forming gears, it can not meet the demand for reducing the weight of the jack. Further, although there has also been used a flange-reinforced thin plate material formed into the shape of gear teeth, the plate material undergoes severe fabrication upon forming the gear and, as a result, the material is degraded to reduce the strength, as well as high accuracy can not be obtained with ease in this case.
In view of the above, rolling contact means that utilize paired cams brought into contact with each other have been employed in recent years.
As such cam contact means, the present inventor has already developed an arm having a first cam of a heart-shaped curved face as an effective cam face which is formed at one of the side plate edges and a second cam of the same heart-shaped curved face as an effective can face which is formed in a different direction to the other of the side plate edges, respectively, of an arm shaft bearing member bended into a U-shaped cross section and has already filed an invention relating to such an arm as Japanese Patent Application No. 166343/1982. In the rolling contact means of this prior application, paired cam faces formed on both of the parallel side edges of one arm that is pivoted to one of shaft holes are brought into rolling contact with the identically shaped opposing paired cam faces of another arm that is pivoted to the other of the shaft holes. However, since each of the opposed cam faces is in a heart-shaped curved face with a small radius of curvature, no intimate engagement can be obtained between them thus failing to obtain satisfactory frictional resistance upon contact. Particularly, since the cam contact becomes insufficient in the angular range between the opposed arms, which is actually used most frequently, load balance between each of the arms is lost thereby causing dangers.